Develop Reference

Why does this code throw ReferenceError: test is not defined?

var te‌st = 1
console.log(test)
Try running this simple code. It gives error: ReferenceError: test is not defined, despite the fact that I defined that variable. Why is this happening?

In the variable declaration, the variable name contains a zero-width non-joiner (ZWNJ) character (between e and s), which is invisible, because its width is equal to zero. However, ECMAScript specification allows this character as a part of variable name.
However, in the console.log() call, there is just test, without any special characters. Therefore, it throws Reference Error, because the variable name is te<ZWNJ>st, not test.
Fortunately, there's an easy way to check if a variable name contains such characters. You can paste your code into JS Bin or JS Fiddle — they denote these characters with a white dot on a red background. That's how it looks like in JS Fiddle:
I think there are also similar features in some IDEs.
Side note: this is an interesting way to prevent people from copy pasting the code snippets you use in answers into their own code. Consider the following code snippet:
// Warning: non-copy-pastable, it won't work if you copy it into your code.
function a‌dd(a, b) {
return a + b
}
console.log(a‌dd(2, 3))
There's a ZWNJ character in the function name and the function call, so it works here. However, if someone copied the function into their code and then manually typed console.log(add(3, 4)), it would throw ReferenceError: add is not defined.
Please don't take the above seriously, it's rather a joke than a practical use.
Related
What characters are valid for JavaScript variable names?
No visible cause for “Unexpected token ILLEGAL”

Error : missing new prefix when invoking a constructor

I am trying to create a function in node.js. The following is relevant code, it gives me error when i call function.
function ReplacePlaceholders() {
return 'success';
}
exports.sendMailMsg = function (templateName, receiverEmail, dataPlaceholders) {
ReplacePlaceholders();
}

In node.js, function names are camel cased, and should start with a lowercase character. Starting a function with an uppercase character tells JSHint to consider the function a constructor rather than a method.
This is actually an error being generated by JSHint, but the code will run correctly. The option in JSHint, newcap, which causes this error is actually depreciated, and disabling it is recommended.
The relevant info as to why this option is even in JSHint:
This option requires you to capitalize names of constructor functions. Capitalizing functions that are intended to be used with new operator is just a convention that helps programmers to visually distinguish constructor functions from other types of functions to help spot mistakes when using this.
Not doing so won't break your code in any browsers or environments but it will be a bit harder to figure out—by reading the code—if the function was supposed to be used with or without new. And this is important because when the function that was intended to be used with new is used without it, this will point to the global object instead of a new object.

The error message you mention is a JSHint error message, not a runtime error. There is a discussion of it here:
jshint expects the new 'prefix' for functions
JSHint expects functions that start with a capital letter to be object definitions. You can ignore the error, disable it in JSHint, or rename your function so that it starts with a lower-case letter.

I'm not sure why that might be, but that error suggests that doing new ReplacePlaceholders(); might work. Though, you may want to consider something like the following:
function ReplacePlaceholders(templateName, receiverEmail, dataPlaceholders) {
return 'success';
}
exports.sendMailMsg = ReplacePlaceholders;

Uncaught SyntaxError: Unexpected token ) when using void()

I get this error and I've managed to narrow it down to:
aaa
That line of code is now the only thing in my source code and still I get the error in title. Any idea why so?
Even when surrounded with the appropriate HTML elements (html, head, body etc) I am still thrown the error. The error shows up in Chrome dev console and via alert if I include a
window.onerror
function in the head tag. It also occurs when the myFunction() method actually exists. As far as I can gather, there is absolutely nothing wrong with that above statement whatsoever.

Use
aaa
void expects a parameter.
There's an interesting discussion on using void(0) or other techniques here.

Is because void takes one argument.
You want:
aaa

void is an operator, not a function. It requires a single expression as its operand. () is not a valid expression. The correct syntax is:
aaa
You can put parentheses around 0, but they're not necessary, just as you don't need parentheses around 0 when writing 3 + 0.

Mysterious line of Javascript code

I am investigating a bug in some software that has uses an in-house developed Javascript library. The error that I am dealing with appears on the line below:
GetVal1("dispLetter")(GetVal1("dispLetter").selectedIndex).value + '~' + (bFinal == true ? '1' : '0');
I initially wasn't sure if this line was even valid, however, according to source control this line was around since this file was created while the error is relatively recent. When I debugged I discovered that this line throws an error that says GetVal1(...) is not a function. I double checked to confirm that the Javascript file with the function definition is included, the header looks like this:
function GetVal1(strHTMLId)
So, I guess my question is, is this line valid Javascript code? Is there anything you can tell that could be throwing the error? Thank you.

GetVal1("dispLetter")(GetVal1("dispLetter").selectedIndex).value + ...
does the following:
calls GetVal1 with the argument "dispLetter".
calls GetVal1 with the argument "dispLetter", again.
retrieves the property selectedIndex of the return value of the second invocation of GetVal1
Calls the return value of the first invocation of GetVal1, with one argument, the value of selectedIndex. This fails your case, and complains the value is not callable.
The return value's value property is dereferenced. String concatenation follows.
In other words, this code seems to assume that the first invocation of GetVal1("dispLetter") returns a function (which is unusual), and the second invocation returns an object with the property selectedIndex (which is unusual, given the first invocation returns a function).
Some ideas:
If there used to be a new keyword before the line. Then the first invocation would be a constructor call. It is unexpected that a constructor call would return a function while a non-constructor call would not, though.
If there used to be a trailing period on the previous line (or is now), GetVal1 would refer (or refers now) to a property of some object. I smell a violation of naming conventions, though, if GetVal1 is meant to be an object property.
The global GetVal1 is (or recently ceased to be) shadowed by a function of the same name. Once again, I smell a violation of naming conventions.
Most likely, GetVal1 itself has changed. Verify GetVal1 can return a function when given this string as the first argument.
Perhaps the state bound to the GetVal1 function has changed (say, one more extra call somewhere before the code. This most likely a design error, though, if this function returns a different type of object on each invocation with the same arguments. But then again, there likely is a design error or naming violation somewhere in the code.
Another plausible explanation is that this line was there from the beginning, but it was never reached before. In this case, it could have been wrong the whole time.

How does Javascript handle function parameters

I recently had an issue with some javascript that goes against every bone of my programming background. Javascript does this often to me, so I'm not that surprised.
I have a function as such...
function x(param1, booleanParam, arrayParam){
....
}
I was getting a runtime error saying that arrayParam.length was not defined. On debugging I saw this was true and went to find out why. Turns out I had forgotten a comma in my function call as such...
x(param1, true [arrayJunk]);
The problem I'm having is figuring out why this call was made at all? Why isn't this a compile error, how does Javascript see this and think, "Yeah, that seems like it might work!"
Thanks in advance for any enlightenment you can share!

That's an indexing expression.
It's the same syntax as someArray[someIndex].
It will end up passing undefined as the second parameter too, unless arrayJunk happens to be the name of a property of boolean primitives.

What happens is the following:
JavaScript engine converts true into a Boolean object (not the primitive)
It then tries to access the property name stored in arrayParam from that object
Property doesn't exist, so it returns undefined
If arrayParam was the string "toString", it would return a function object

In this case the expression was being interpreted as an index. Essentially the same as
someArray[42]
So it was being seen as a function call with 2 parameters instead of 3

Many dynamic languages don't check if you pass too many or too few arguments to a function.
While this can sometimes mask errors, it also allows you to roll your own defalut parameter scheme.